1. Field of the Invention
The invention relates to a rim, or to a portion of a rim, made of composite material.
2. Background Information
Initially, bicycle wheels were manufactured mostly out of metals such as steel and aluminum. A conventional method for manufacturing a metallic rim is to make a straight profiled section, to bend it, and then to weld its two ends together to obtain a hoop. Such a method is possible only because metallic materials have sufficient ductility.
Nowadays, rims are also made of composite materials. The rims, or portions thereof, made of composite material are very popular with cyclists for the stiffness/weight ratio that they offer.
A wide range of options are available in choosing the materials and the method for manufacturing a composite rim. For example, one can use glass, carbon, or aramid fibers, embedded in a matrix that can be either thermoplastic or thermosetting. In addition, one can choose fibers of various lengths, which can be arranged with respect to one another in various configurations.
For example, a combination of short fibers within a thermoplastic matrix enables a rim to be manufactured by injection. Such a method does not produce a rim element having a very good stiffness/weight ratio, particularly because the fibers used must be very short and the ratio of the quantity of fibers to the quantity of thermoplastic matrix is too small to allow full benefit from the advantages provided by the fibers.
One can also envision using woven fibers within a thermoplastic matrix to enable a manufacturing method similar to that used for metallic rims, i.e., making a rectilinear shaped element that is then bent under heat. Of course, such a method requires the use of a thermoplastic matrix. Moreover, as the glass or carbon fibers have a very high modulus and cannot stretch, such a method also requires a particular configuration for the fiber orientation. Typically, during the bending process it is impossible to bend an element whose fibers, positioned in a portion of such element, are oriented in a direction of the element corresponding to such portion and to the elongation direction. In addition, the deformation of a rectilinear shaped element for forming a circular hoop requires the fibers, which are non-extensible, to be small in quantity relative to the that of the matrix. Finally, obtaining a rim with a hollow profile using such a method cannot be envisioned.
It is now known that to make a composite rim with a very good stiffness/weight ratio, long fibers in large quantities should preferably be used and they should be positioned along all directions, particularly along a circumferential direction within a thermosetting matrix.
The manufacture of the aforementioned rims is expensive because of the cost of the materials, but also because of the significant amount of time required for their manufacture.
U.S. Pat. No. 6,347,839 describes a method for manufacturing a composite rim. The rim is comprised of twenty-four layers containing fibers, i.e., glass fibers. The various epoxy resin-impregnated layers are positioned in a mold on top of one another, thus forming a laminated fabric. After polymerization of the resin, the rim is extracted from the mold. A rim is advantageously made in a single piece, but the process is time-consuming and requires much manipulation. In addition, automating the positioning of the numerous layers in the mold is impossible. Therefore, the presence of an operator is required during the entire cycle for preparing a single rim.
Furthermore, this method makes it impossible to mold hollow elements, such as bicycle rims. Therefore, to overcome this difficulty, inner cores made of very light material are used, which remain in place once the rim is manufactured. Of course, such a solution is not optimal in terms of weight. In addition, to avoid crushing the core, which is often made of foam, the resin injection pressure is limited. Another solution to overcome this difficulty is to make the rim into a plurality of portions having annular and open shapes, which are glued to one another to form the final rim. This solution not only is costly in terms of time and materials, but is also inefficient because the rims thus made are heavier.
Another known method for making a rim is to use of an inflatable bladder. This method eliminates the need for the core, but remains a long and expensive process.
U.S. Pat. No. 6,398,313 describes another method in which the rim is comprised of two half-rims, each of which is a section having an angular arc of 180°. Each of the sections is made by molding. Due to this two-hemisphere configuration, the closed cross-sectional portions of the rim are now hollow portions of the sections, which are open at both ends of the sections. For the molding, it is therefore possible to use a harder, extractable core, and higher pressures are possible. The two junctions between the sections are made by positioning sleeves within the rim. Due to the presence of these two sleeves, this technology does not make it possible to produce weight-optimized rims.
Furthermore, this method remains a long and costly process. Indeed, as is the case with all methods for molding a composite structure comprised of various layers, the positioning of the layers in the mold generates a large amount of waste because the layers are cut from fabric panels of standard size prior to being inserted into the mold. In addition, the molding processes also generate a waste of resin that is difficult to recycle as it is already polymerized. Therefore, in addition to the disadvantages related to the cost and the manufacturing time, molding a composite rim also poses environmental problems.